Download - Evidence-based diagnosis
Evidence Based Diagnosis
When a Patient Has a ProblemWhen a Patient Has a Problem
The doctor reaches a diagnosis by:The doctor reaches a diagnosis by:
• Clinical dataClinical data
• Diagnostic toolsDiagnostic tools
Increasing use of Diagnostic Increasing use of Diagnostic tests:tests:
- Availability.- The urge to make use of new technology.
The evaluation of diagnostic techniques is less advanced than that
of treatments (NO phase I, II, III, IV).
New Diagnostic testsNew Diagnostic tests
RelevanceRelevance
• First, the test should be one that is feasible for you in your community
• Example: brain biopsy is an accurate test for diagnosing dementia, it’s not practical for my (living) patients!
• Can I apply the test to my patients? (Availability, Cost) e.g MRI
ValidityValidity
The degree to which the results of a study are likely to be true and free from bias.
• It should be compared to a gold reference standard
CautionCaution
• reference standard used should be acceptable (e.g HSG vs DL)
• Both reference standard and test should be applied to all patients
IndependentIndependent
• the decision to perform the reference standard should ideally be independent of the results of the test being studied.
Ask yourselfAsk yourself
• the patient sample should include an appropriate spectrum of patients to whom the diagnostic test will be applied in clinical practice
Rule of ThumbRule of Thumb
• at least 100 participants to ensure an appropriate "spectrum" of disease
22 x 2 tablex 2 table comparing the results of a diagnostic test with a reference standard
reference standard
diseaseno disease
test abnormaltrue pos. [a]false pos. [b]
test normalfalse neg. [c]true neg. [d]
sensitivitysensitivity
• probability of a positive test among patients with disease
• i.e Ability to diagnose
specificityspecificity
• probability of a negative test among patients without disease
• i.e Ability to exclude
22 X 2 TableX 2 Table
a(true positive)
b(false positive)
c(false negative)
d(true negative)
Keep in MindKeep in Mind
• sensitivity and specificity by themselves are only useful when either is very high (over typically, 95% or higher).
Who wants whatWho wants what? ?
main interest
Methodologistsensitivity
specificity
Doctoraccuracy
PatientProbability
Likelihood RatioLikelihood Ratio
The "positive likelihood ratio" (LR+) tells us how much to increase the probability of disease if the test is positive
The "negative likelihood ratio" (LR-) tells us how much to decrease it if the test is negative
Likelihood RatioLikelihood RatioLR=+
probability of a +ve test in those who have the disease___probability of a +ve test in those who do not have the disease
= sensitivity 1-specificity
LR=- probability of a -ve test in those who have the disease___
probability of a -ve test in those who do not have the disease
= 1-sensitivity specificity
LRInterpretationInterpretation >10
Large and often conclusive increase in the likelihood of disease
5 - 10Moderate increase in the likelihood of disease
2 - 5Small increase in the likelihood of disease
1 - 2Minimal increase in the likelihood of disease
1No change in the likelihood of disease
0.5 - 1.0
Minimal decrease in the likelihood of disease
0.2 - 0.5
Small decrease in the likelihood of disease
0.1 - 0.2
Moderate decrease in the likelihood of disease
<0.1Large and often conclusive decrease in the likelihood of
disease
Why LR
• The LR+ corresponds to the clinical concept of "ruling-in disease"
• The LR- corresponds to the clinical concept of "ruling-out disease“
Patient oriented!!!!!!!
• Your 45 year old patient has a mammogram. The study is interpreted as "suspicious for malignancy" by your radiologist.
• Your patient asks you:"Does this mean I have cancer?", and you (correctly) answer "No, we have todo further testing."
• Your patient then asks, "OK, I understand that the mammogram isn't the final answer, but given what we know now, what are the chances that I have breast cancer?".
Is it Easy!!!
• Assume that the overall risk of breast cancer in any 45 year old woman,
regardless of mammogram result, is1%. Assume also that mammography is 90% sensitive and 95% specific. Then,
select your answer below:
1% 15% 60% 85% 95%
If you know that the risk of breast cancer in any 45 year old woman is 1% and that mammography is 90% sensitive and 95% specific.
What do you think your patient’s probability of having breast cancer is?
LR+=Sens/100-Spec
=90/5=18
Diseaseruled IN
Disease ruled OUT
Disease not
ruled in or out
Above this point,treat
Below this point,no further testing
Determined by:Complications of untreated disease
Risks of therapyComplications of tests
Cost
ROC curve is simply a graph of sensitivity vs (1-specificity)
ScoreSystematic Collaboration of Ovarian Reserve Evaluation
systematic reviews of Diagnostic tests
THANK YOU